Didier Stevens

Sunday 29 November 2020

Update: emldump.py Version 0.0.11

Filed under: My Software,Update — Didier Stevens @ 21:50

This is the Python 3 version of my email file analysis tool (eml).

emldump_V0_0_11.zip (https)
MD5: 09408ED0C2183178BEA71459CE001995
SHA256: 01B3543CCBAE806E1536BF55E62DF7D30885737909DB4322348AC521138660CC

Saturday 28 November 2020

Update: disitool.py Version 0.4

Filed under: My Software,Update — Didier Stevens @ 11:08

This is a Python 3 update for my disitool.

disitool_v0_4.zip (https)
MD5: 3A41D8805340716913FAECE7C79B10A7
SHA256: 51EBFB0759FEEA69FFFB643659FD74DC5043338719A91CE36E427D175196661A

Wednesday 18 November 2020

Decrypting With translate.py

Filed under: Encryption,My Software — Didier Stevens @ 0:00

You’ve probably encountered malicious PowerShell scripts with an encrypted payload (shellcode, PowerShellScript, …).

Here is an example that I created:

Update: this example is on pastebin: https://pastebin.com/QUGiWTHj

There are 2 BASE64 strings in this script. The first one (cfr. variable $cfii) is the encryption key. The second one (cfr. variable $hctqdvb) is the payload.

The script uses AES encryption, with a 256-bit key, CBC mode, PKCS7 padding and an initialization vector (IV) that is stored in the first 16 bytes of the payload (0..15).

And after the payload is decrypted, it has to be decompressed with the Gzip algorithm.

With base64dump.py, I can find the 2 BASE64 strings in the PowerShell script:

I select the second BASE64 string (payload) to pipe into translate.py, using the following small Python script (decrypt.py) to do the decryption:

from Crypto.Cipher import AES
from Crypto.Util import Padding

def Decrypt(data):
    iv = data[0:16]
    ciphertext = data[16:]
    key = binascii.a2b_base64(keybase64)
    oAES = AES.new(key, AES.MODE_CBC, iv)

return Padding.unpad(oAES.decrypt(ciphertext), 16)

This small script uses crypto functions from pycryptodome.

I use translate.py in fullread mode (-f –fullread, to “translate” the file in a single step, in stead of byte per byte) and use function Decrypt to decrypt the block of data, like this:

I load the script decrypt.py with option -s, and I pass the key as a BASE64 string via option -e.

The output is non-printable bytes, because the decrypted payload is Gzip compressed. I use translate.py again to do the decompression:

And now the “payload” I used is decrypted and decompressed: “This is a test!”

 

Sunday 15 November 2020

oledump Indicators

Filed under: maldoc,My Software — Didier Stevens @ 13:51

Each stream and storage can have an indicator in oledump.py‘s output:

You’ll probably know M and m: they are indicators that appear often.

Here is an overview of all possible indicators:

  • M: Macro (attributes and code)
  • m: macro (attributes without code)
  • E: Error (code that throws an error when decompressed)
  • !: Unusual macro (code without attributes)
  • O: object (embedded file)
  • .: storage
  • R: root entry

 

Update: oledump.py Version 0.0.55

Filed under: My Software,Update — Didier Stevens @ 13:49

This new version of oledump.py brings extra JSON support and a new indicator.

Existing option -j (–jsonoutput) produces JSON output: a JSON object with the content of each individual stream (BASE64 encoded).

This option (-j) can now be used together with option -v (–vbadecompress) to produce a JSON object with the VBA code (BASE64 encoded) of each VBA module stream.

And there is a new indicator (!) :

This indicator is used for VBA module streams for which oledump is not able to recognize “normal” VBA source code (e.g. starting with something else than attributes). Here is an example of a sample that would cause this ! indicator to appear: AV Cleaned Maldoc.

oledump_V0_0_55.zip (https)
MD5: 499B66DC3BAF86BDA4BC0370E3C18A1A
SHA256: ABEABFF0F1F5AA2239AFCDE73A676D4E8D9BA2F82C03B8663FFAB6F8D3A360E7

Wednesday 11 November 2020

Update: translate.py Version 2.5.10

Filed under: My Software,Update — Didier Stevens @ 0:00

This is a Python 3 bug fix version.

translate_v2_5_10.zip (https)
MD5: DB9574D664257263C51FE7C74C7B281E
SHA256: E8993B3F2C25A92A9F4583636E1CEF79D79649B29FFF56EAA9AF8A30FCF9B9A6

Saturday 7 November 2020

1768 K

Filed under: My Software,Reverse Engineering — Didier Stevens @ 0:00

According to Wikipedia, 1768 Kelvin is the melting point of the metal cobalt.

This tool decodes and dumps the configuration of Cobalt Strike beacons.

You can find a sample beacon here.

1768_v0_0_3.zip (https)
MD5: 73DB2E96EE5B6427AF6CCE2672F91CB2
SHA256: C06850A132B89F5E8C127E43FD5CC42051706CDF058EB2D688BC8BD3043E6E02

Thursday 22 October 2020

Update: strings.py Version 0.0.5 Pascal Strings

Filed under: My Software,Update — Didier Stevens @ 0:00

This new version of strings.py, my tool to extract strings from arbitrary files, adds option -P to add support for Pascal strings.

A Pascal string is a string that is internally stored with a length-prefix: an integer that counts the number of characters inside the string.

The Unix strings command, and my strings.py tool, can extract Pascal strings without any problem, because they just search for a sequence of characters, without looking for a terminating NULL character (C-string) or a length-prefix (P-string ot Pascal string).

But with option -P, you can direct my tool strings.py to only extract Pascal strings, by checking if character sequences are prefixed with an integer that is equal to the number of characters inside the string. Strings that do not match that requirement are ignored.

Since an integer can be represented internally with different byte formats, you have to provide a value to option -P that indicates how the integer is stored internally. I use the same format as Python’s struct module to represent that format. For example, “<I” is a little-endian, unsigned 32-bit integer. That is how a string is represented in Delphi, as can be seen in this example of a Delphi malware sample:

The strings you see here are all found inside the sample, and are prefixed by their length. If you wouldn’t use option -P, then these strings would also be extracted, but they would not stand out amid the other strings that are not prefixed by their length.

Delphi also supports the ShortString type: one byte to encode the length. These can be found with option -P “<B”: little-endian, unsigned 8-bit integer:

strings_V0_0_5.zip (https)
MD5: A4BF314BE0A72972ECA7B14B558610E6
SHA256: 30E9E9BB618006445483AA78F804766D8FFA518974B81F9B68FF534BEA30B072

Sunday 18 October 2020

Update: translate.py version 2.5.9

Filed under: My Software,Update — Didier Stevens @ 0:00

This is a small bug fix release for Python 3.

translate_v2_5_9.zip (https)
MD5: 8EC7A9F0738C86CCF2F0B44D3994E798
SHA256: 3C469996F7014CC1BD5D4F02157B7D5803698D93018360904B79EA2A1601BD10

Tuesday 6 October 2020

Update: oledump.py Version 0.0.54

Filed under: My Software,Update — Didier Stevens @ 0:00

This new version of oledump.py adds a new variable for option -E: %MOFULEINFO%

This variable need to be used together with option -i: it contains the size of the compiled VBA code and the compressed VBA code. For example: 123+65.

There’s a new option (-s) for plugin plugin_http_heuristics: with this option, the plugin ignores space characters (useful for hexadecimal bytes separated by a space character, for example).

And there is a new plugin: plugin_msg_summary. This is a new type of plugin, a plugin that operates on the complete document. Before, plugins could only operate on individual streams, and were instantiated for each stream.

This plugin produces a summary of a .msg file (something we needed for our “Epic Manchego” research).

Here is an example:

This plugin has a couple of options, for example to produce JSON output or to add header or body information:

 

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